Scattered pilot pattern and channel estimation method for MIMO-OFDM systems

ABSTRACT

A method and apparatus are provided for reducing the number of pilot symbols within a MIMO-OFDM communication system, and for improving channel estimation within such a system. For each transmitting antenna in an OFDM transmitter, pilot symbols are encoded so as to be unique to the transmitting antenna. The encoded pilot symbols are then inserted into an OFDM frame to form a diamond lattice, the diamond lattices for the different transmitting antennae using the same frequencies but being offset from each other by a single symbol in the time domain. At the OFDM receiver, a channel response is estimated for a symbol central to each diamond of the diamond lattice using a two-dimensional interpolation. The estimated channel responses are smoothed in the frequency domain. The channel responses of remaining symbols are then estimated by interpolation in the frequency domain.

RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.10/038,883 filed Jan. 8, 2002, now U.S. Pat. No. 7,248,559 and claimsthe benefit thereof, which itself claims the benefit of U.S. ProvisionalApplication No. 60/329,509 filed Oct. 17, 2001, the contents of whichare incorporated in its entirety herein by reference.

FIELD OF THE INVENTION

This invention relates to OFDM communication systems, and moreparticularly to a more efficient use of pilot symbols within suchsystems.

BACKGROUND OF THE INVENTION

Multiple Input Multiple Output—Orthogonal Frequency DivisionMultiplexing (MIMO-OFDM) is a novel highly spectral efficient technologyused to transmit high-speed data through radio channels with fast fadingboth in frequency and in time.

In wireless communication systems that employ OFDM, a transmittertransmits data to a receiver using many sub-carriers in parallel. Thefrequencies of the sub-carriers are orthogonal. Transmitting the data inparallel allows the symbols containing the data to be of longerduration, which reduces the effects of multi-path fading. Theorthogonality of the frequencies allows the sub-carriers to be tightlyspaced, while minimizing inter-carrier interference. At the transmitter,the data is encoded, interleaved, and modulated to form data symbols.Overhead information is added, including pilot symbols, and the symbols(data plus overhead) are organized into OFDM symbols. Each OFDM symboltypically uses 2^(n) frequencies. Each symbol is allocated to representa component of a different orthogonal frequency. An inverse Fast FourierTransform (IFFT) is applied to the OFDM symbol (hence the preference of2^(n) frequencies) to generate time samples of a signal. Cyclicextensions are added to the signal, and the signal is passed through adigital-to-analog converter. Finally, the transmitter transmits thesignal to the receiver along a channel.

When the receiver receives the signal, the inverse operations areperformed. The received signal is passed through an analog-to-digitalconverter, and timing information is then determined. The cyclicextensions are removed from the signal. The receiver performs an FFT onthe received signal to recover the frequency components of the signal,that is, the data symbols. Error correction may be applied to the datasymbols to compensate for variations in phase and amplitude causedduring propagation of the signal along the channel. The data symbols arethen demodulated, de-interleaved, and decoded, to yield the transmitteddata.

In systems employing differential detection, the receiver compares thephase and/or amplitude of each received symbol with an adjacent symbol.The adjacent symbol may be adjacent in the time direction or in thefrequency direction. The receiver recovers the transmitted data bymeasuring the change in phase and/or amplitude between a symbol and theadjacent symbol. If differential detection is used, channel compensationneed not be applied to compensate for variations in phase and amplitudecaused during propagation of the signal. However, in systems employingcoherent detection the receiver must estimate the actual d phase andamplitude of the channel response, and channel compensation must beapplied.

The variations in phase and amplitude resulting from propagation alongthe channel are referred to as the channel response. The channelresponse is usually frequency and time dependent. If the receiver candetermine the channel response, the received signal can be corrected tocompensate for the channel degradation. The determination of the channelresponse is called channel estimation. The inclusion of pilot symbols ineach OFDM symbol allows the receiver to carry out channel estimation.The pilot symbols are transmitted with a value known to the receiver.When the receiver receives the OFDM symbol, the receiver compares thereceived value of the pilot symbols with the known transmitted value ofthe pilot symbols to estimate the channel response.

The pilot symbols are overhead, and should be as few in number aspossible in order to maximize the transmission rate of data symbols.Since the channel response can vary with time and with frequency, thepilot symbols are scattered amongst the data symbols to provide ascomplete a range as possible of channel response over time andfrequency. The set of frequencies and times at which pilot symbols areinserted is referred to as a pilot pattern. The optimal temporal spacingbetween the pilot symbols is usually dictated by the maximum anticipatedDoppler frequency, and the optimal frequency spacing between the pilotsymbols is usually dictated by the anticipated delay spread ofmulti-path fading.

The existing pilot-assisted OFDM channel estimation approaches aredesigned for conventional one transmitter system. With a scattered pilotarrangement, there are three classes of algorithms:

-   -   1-D frequency interpolation or time interpolation    -   Transformed frequency 1-D interpolation    -   Independent time and frequency 1-D interpolation

The first class of algorithms is based on the pilot OFDM symbol (all thesub-carriers are used as the pilots) or comb-type of pilots. Thisapproach shown in the flow chart of FIG. 1A is simple but only suitablefor channels with high frequency selectivity or channels with high timefading. The method involves pilot extraction in the frequency domain(step 1A-1) followed by interpolation in time (step 1A-2), orinterpolation in frequency (step 1A-3).

The second method shown in the flow chart of FIG. 1B is aimed forchannels with slow Doppler fading and fast frequency fading. It improvesthe first method by using FFT to reconstruct the channel response backto time domain for noise reduction processing at the expense of FFT/IFFTcomputing for the channel estimation separately. The method begins withpilot extraction in the frequency domain (step 1B-1), which may befollowed by interpolation in frequency (step 1B-2). Then an inverse fastFourier transform (step 1B-3), smoothing/de-noise processing (step1B-4), and finally a fast Fourier transform (1B-5) steps are executed.

The third method shown in the flow chart of FIG. 1C can be used toestimate channel for mobile applications, where both fast time fadingand frequency fading exist. However it needs a relatively high densityof pilots and a completed interpolator. This method involves pilotextraction in the frequency domain (step 1C-1) this is followed byinterpolation in time (step 1C-2) and interpolation in frequency (step1C-3).

In the propagation environment with both high frequency dispersion andtemporal fading, the channel estimation performance can be improved bythe increase of pilot symbol density at the price of the reduction ofthe spectral efficiency of the data transmission. To interpolate andreconstruct the channel response function from the limited pilots toachieve reliable channel estimation with the minimum overhead is achallenging task.

There are a variety of existing standard pilot patterns. In environmentsin which the channel varies only slowly with time and frequency, thepilot symbols may be inserted cyclically, being inserted at an adjacentfrequency after each time interval. In environments in which the channelis highly frequency dependent, the pilot symbols may be insertedperiodically at all frequencies simultaneously. However, such a pilotpattern is only suitable for channels that vary very slowly with time.In environments in which the channel is highly time dependent, the pilotsymbols may be inserted continuously at only specific frequencies in acomb arrangement to provide a constant measurement of the channelresponse. However, such a pilot pattern is only suitable for channelsthat vary slowly with frequency. In environments in which the channel isboth highly frequency and highly time dependent (for example, mobilesystems with much multi-path fading), the pilot symbols may be insertedperiodically in time and in frequency so that the pilot symbols form arectangular lattice when the symbols are depicted in a time-frequencydiagram.

In OFDM communication systems employing coherent modulation anddemodulation, the receiver must estimate the channel response at thefrequencies of all sub-carriers and at all times. Although this requiresmore processing than in systems that employs differential modulation anddemodulation, a significant gain in signal-to-noise ratio can beachieved using coherent modulation and demodulation. The receiverdetermines the channel response at the times and frequencies at whichpilot symbols are inserted into the OFDM symbol, and performsinterpolations to estimate the channel response at the times andfrequencies at which the data symbols are located within the OFDMsymbol. Placing pilot symbols more closely together (in frequency if acomb pattern is used, in time if a periodic pattern is used, or in bothfrequency and in time if a rectangular lattice pattern is used) within apilot pattern results in a more accurate interpolation. However, becausepilot symbols are overhead, a tighter pilot pattern is at the expense ofthe transmitted data rate.

Existing pilot patterns and interpolation techniques are usuallysufficient if the channel varies slowly with time (for example fornomadic applications). However, if the channel varies quickly with time(for example, for mobile applications), the time interval between pilotsymbols must be reduced in order to allow an accurate estimation of thechannel response through interpolation. This increases the overhead inthe signal.

The problem of minimizing the number of pilot symbols while maximizingthe accuracy of the interpolation is also particularly cumbersome inMultiple-Input Multiple-Output (MIMO) OFDM systems. In MIMO OFDMsystems, the transmitter transmits data through more than onetransmitting antenna and the receiver receives data through more thanone receiving antenna. The binary data is usually divided between thetransmitting antennae, although the same data may be transmitted througheach transmitting antenna if spatial diversity is desired. Eachreceiving antenna receives data from all the transmitting antennae, soif there are M transmitting antennae and N receiving antennae, then thesignal will propagate over M×N channels, each of which has its ownchannel response. Each transmitting antenna inserts pilot symbols intothe same sub-carrier location of the OFDM symbol which it istransmitting. In order to minimize interference at the receiver betweenthe pilot symbols of each transmitting antenna, each transmittingantenna typically blinks its pilot pattern on and off. This increasesthe temporal separation of the pilot symbols for each transmitter,reducing the accuracy of the interpolation used to estimate the channelresponse. In MIMO-OFDM systems a simple and fast channel estimationmethod is particularly crucial because of the limitation of thecomputational power for estimating M×N channels, while in SISO-OFDMsystem only one channel needs to be estimated.

SUMMARY OF THE INVENTION

Channel estimation methods are provided which are based on the partialinterpolation of a scattered pilot by using true 2-D interpolation; andadditionally, simple 1-D interpolation is used reconstruct the entirechannels. This method has a reduced scattered pilot overhead, and is atleast an order of magnitude less computationally complex than someexisting methods. In general, the proposed method of channel estimationis more robust in channels with high Doppler spread, and provides betterperformance than some existing methods and requires the less bufferingof the OFDM symbols for the coherent detection at the receiver than insome methods.

The methods allow fewer pilot symbols to be placed within each OFDMsymbol, while still allowing accurate interpolation of the channelresponse. The data rate of an MIMO-OFDM system is thereby improved.

According to a first aspect of the invention, there is provided a methodof inserting pilot symbols into Orthogonal Frequency DivisionMultiplexing (OFDM) frames transmitted on a plurality N of transmittingantenna, the OFDM frames having a time domain and a frequency domain,each OFDM frame comprising a plurality of OFDM symbols, the methodcomprising the steps of: for the N transmit antennas, transmitting setsof N pilot symbols, each set being in a location within a scatteredpattern in time-frequency, each set of N pilot symbols comprising apilot symbol for each antenna.

In some embodiments, transmitting sets of N pilot symbols, each setbeing in a location within a scattered pattern in time-frequencycomprises: transmitting a set of N pilot symbols in a respectivelocation within the scattered pattern on a same sub-carrier.

In some embodiments, for the N transmit antennas, transmitting sets of Npilot symbols, each set being in a location within a scattered patternin time-frequency comprises: inserting sets of N pilot symbols atlocations that form at least one diagonal arrangement in time-frequency.

In some embodiments, inserting sets of N pilot symbols comprises: when Nis equal to two, for each antenna, alternating insertion of null symbollocations and pilot symbols in the at least one diagonal arrangement fora first antenna of the pair of antennas and alternating insertion ofpilot symbols and null symbol locations in the at least one diagonalarrangement for a second antenna of the pair of antennas, wherein thenull symbol locations of the first antenna correspond to a same locationin time-frequency as the pilot symbols of the second antenna, and viceversa.

In some embodiments, the method further comprises for each locationwithin a scattered pattern in time-frequency: generating a group of Luncoded pilot symbols; performing space time block coding (STBC) on thegroup of L uncoded pilot symbols to produce an N×N STBC block, L and Ndetermining an STBC code rate; transmitting one row or column of theSTBC block on each antenna on a specific sub-carrier.

In some embodiments, the method further comprises transmitting the setsof N pilot symbols with a power level greater than a power level of datasymbols, depending upon a value reflective of channel conditions.

In some embodiments, the method further comprises transmitting the setsof N pilot symbols with a power level which is dynamically adjusted toensure sufficiently accurate reception as a function of a modulationtype applied to sub-carriers carrying data.

In some embodiments, transmitting sets of N pilot symbols, each setbeing in a location within a scattered pattern in time-frequencycomprises: providing a first plurality of equally spaced sub-carrierpositions; providing a second plurality of equally spaced sub-carrierpositions offset from said first plurality; inserting the sets of Npilot symbols alternately in time using the first plurality of equallyspaced sub-carrier positions and the second plurality of equally spacedsub-carrier positions.

In some embodiments, the second plurality of equally spaced sub-carrierpositions is offset from the first plurality of equallyspaced-subcarrier positions by half the spacing between adjacentsub-carriers of the first plurality of sub-carrier positions therebyforming a diamond shaped arrangement.

In some embodiments, the method further comprises inserting sets of Npilot symbols in an OFDM resource for an additional group of Ntransmitting antennas wherein transmitting sets of N pilot symbols in arespective pattern in time-frequency for the additional group of Ntransmitting antennas comprises: employing the same respective patternof pilot symbols as the N transmitting antennas where N≧2, but offset inat least one of time and frequency.

According to a second aspect of the invention, there is provided amethod comprising: providing a first transmitter implementing the methodaccording to the first aspect of the invention; providing at least oneother transmitter implementing the method according to the first aspectof the invention using scattered patterns offset from those used by thefirst transmitter.

According to a third aspect of the invention, there is provided atransmitter comprising: a plurality N of transmit antennas; an OFDMframe generator that inserts pilot symbols into Orthogonal FrequencyDivision Multiplexing (OFDM) frames transmitted on the plurality N oftransmit antennas, the OFDM frames having a time domain and a frequencydomain, each OFDM frame comprising a plurality of OFDM symbols, suchthat for the N transmit antennas, sets of N pilot symbols aretransmitted, each set being in a location within a scattered pattern intime-frequency, each set of N pilot symbols comprising a pilot symbolfor each antenna.

In some embodiments, a set of N pilot symbols in a respective locationwithin the scattered pattern is transmitted on a same sub-carrier.

In some embodiments, the transmitter is further operable to, for eachlocation in the scattered pattern: generate a group of L uncoded pilotsymbols; perform space time block coding (STBC) on the group of L pilotsymbols to produce an N×N STBC block; transmit one row or column of theSTBC block on each antenna.

In some embodiments, the transmitter is further operable to transmit thesets of N pilot symbols with a power level greater than a power level ofdata symbols depending on a value reflective of channel conditions.

In some embodiments, the transmitter is further operable to transmit thesets of N pilot symbols with a power level which is dynamically adjustedto ensure sufficiently accurate reception.

In some embodiments, the OFDM frame generator is operable to: define afirst plurality of equally spaced sub-carrier locations; define a secondplurality of equally spaced sub-carrier locations offset from said firstplurality; wherein the sets of N pilot symbols are inserted alternatelyin time using the first plurality of equally spaced sub-carrierlocations and the second plurality of equally spaced sub-carrierlocations.

In some embodiments, spacing between locations of the scattered patternin time-frequency is optimized to allow a fast extraction of scatteredpilot symbols without requiring the computation of a complete FFT.

According to a fourth aspect of the invention, there is provided areceiver comprising: a plurality N of receive antennas for receivingOFDM symbols comprising: sets of N pilot symbols transmitted from Nantennas in a scattered pattern in time-frequency, the sets of N pilotsymbols for each respective pattern in time-frequency inserted such thatsets of N pilot symbols from different antennas do not occupy a samelocation in time-frequency; and data symbols in time-frequency; and achannel estimator for comparing the received sets of N pilot symbolswith pilot symbol values known to be transmitted by a transmitter.

Other aspects and features of the present invention will become apparentto those ordinarily skilled in the art upon review of the followingdescription of specific embodiments of the invention in conjunction withthe accompanying Figures.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in greater detail with reference tothe accompanying Figures, in which:

FIG. 1 illustrates flow-charts for three examples of conventional OFDMChannel Estimation;

FIG. 2 is a block diagram of a Multiple-Input Multiple-Output OrthogonalFrequency Division Multiplexing (OFDM) transmitter provided by anembodiment of the invention;

FIG. 3 is a block diagram of an OFDM receiver;

FIG. 4 is a flowchart of a method by which an OFDM transmitter insertspilot symbols into an OFDM frame according to one embodiment of theinvention;

FIG. 5 is a diagram of a pilot pattern generated using the method ofFIG. 4;

FIG. 6 is a block diagram of a MIMO system showing the channel transferfunctions between two transmit antennas and two receive antennas;

FIG. 7 is a time frequency diagram showing channel estimate positionsfor pilot channel estimation;

FIG. 8 schematically illustrates a step of filtering estimated andinterpolated pilot channel estimates;

FIG. 9 shows schematically the step of interpolating between the channelestimates previously determined to provide channel estimates for allsub-carriers and all times;

FIG. 10 is a flow chart summarizing the overall channel estimationmethod provided by an embodiment of the invention; and

FIG. 11 is an example of a set of performance results obtained using themethod of FIG. 10.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following sections describe a MIMO-OFDM transmitter/receiver andscattered pilot insertion. By way of introduction, a OFDM frame consistsof the preamble OFDM symbols and regular OFDM symbols. Each OFDM symboluses a set of orthogonal sub-carriers. When there are two transmitantennas, two OFDM symbols form a STTD block. For regular OFDM symbols,some sub-carriers are used as pilot sub-carriers to carry pilot symbolswhile the others are used as data sub-carriers to carry data symbols.The pilot sub-carriers are modulated by pilot symbols generated by QPSK.The data sub-carriers are modulated by complex data symbols generated byQAM mapping. STTD coding is applied to the pilot sub-carrier pairslocated at the same frequency within one STTD block.

Referring to FIG. 2, a block diagram of a Multiple-Input Multiple-Output(MIMO) Orthogonal Frequency Division Multiplexing (OFDM) transmitterprovided by an embodiment of the invention is shown. The OFDMtransmitter shown in FIG. 2 is a two-output OFDM transmitter, thoughmore generally there may be a plurality of M transmitting antennae. AnOFDM transmitter 10 takes binary data as input but data in other formsmay be accommodated. The binary data is passed to a coding/modulationprimitive 12 responsible for encoding, interleaving, and modulating thebinary data to generate data symbols, as is well known to those skilledin the art. The coding/modulation primitive 12 may include a number ofprocessing blocks, not shown in FIG. 2. An encoder 14 applies Space-TimeBlock Coding (SBTC) to the data symbols. The encoder 14 also separatesthe data symbols into a first processing path 16 and a second processingpath 18, by sending alternate data symbols along each of the twoprocessing paths. In the more general case in which the OFDM transmitter10 includes M transmitting antennae, the encoder 14 separates the datasymbols into M processing paths.

The data symbols sent along the first processing path 16 are sent to afirst OFDM component 20. The data symbols are first passed to ademultiplexer 22 in the first OFDM component 20, after which the datasymbols are treated as sub-carrier components. The data symbols are thensent to a pilot inserter 24, where pilot symbols are inserted among thedata symbols. Collectively, the data symbols and pilot symbols arereferred to hereinafter simply as symbols. The symbols are passed to anInverse Fast Fourier Transform (IFFT) processor 26, then to amultiplexer 28 where they are recombined into a serial stream. A guardinserter 30 adds prefixes to the symbols. Finally, the OFDM signals arepassed through a hard limiter 32, a digital-to-analog converter 34, anda radio frequency (RF) transmitter 36 which transmits OFDM symbols as asignal through a first transmitting antenna 37. In most embodiments,each element in the first OFDM component 20 is a processor, a componentof a larger processor, or a collection of processors or any suitablecombination of hardware, firmware and software. These might includegeneral purpose processors, ASICs, FPGAs, DSPs to name a few examples.

The pilot inserter 24 is connected to receive space-time coded pilotsymbols from pilot STBC function 23 which performs STBC on pilot symbols21. The pilot STBC block 23 takes two pilot symbols at a time forexample P₁ and P₂ as indicated in FIG. 2 and generates an STBC blockconsisting of a two by two matrix having (P₁, P₂) in the first row andhaving (−P₂*, P₁*) in the second row. It is the first row of this STBCblock that is inserted by the pilot inserter 24.

The data symbols sent along the second processing path 18 are sent to asecond OFDM component 38 which includes processors similar to thoseincluded in the first OFDM component 20. However, the pilot inserter 40inserts encoded pilot symbols from the second row of the STBC blockproduced by the pilot STBC function 23. The symbols sent along thesecond processing path 18 are ultimately transmitted as a signal througha second transmitting antenna 42.

Referring now to FIG. 3, a block diagram of an MIMO-OFDM receiver isshown. An OFDM receiver 50 includes a first receiving antenna 52 and asecond receiving antenna 54 (although more generally there will be oneor more receiving antennae). The first receiving antenna 52 receives afirst received signal. The first received signal is a combination of thetwo signals transmitted by the two transmitting antennae 37 and 42 ofFIG. 2, although each of the two signals will have been altered by arespective channel between the respective transmitting antenna and thefirst receiving antenna 52. The second receiving antenna 54 receives asecond received signal. The second received signal is a combination ofthe two signals transmitted by the two transmitting antennae 37 and 42of FIG. 2, although each of the two signals will have been altered by arespective channel between the respective transmitting antenna and thesecond receiving antenna 54. The four channels (between each of the twotransmitting antennae and each of the two receiving antennae) may varywith time and with frequency, and will usually be different from eachother.

The OFDM receiver 50 includes a first OFDM component 56 and a secondOFDM component 58 (although in general there will be N OFDM components,one for each receiving antenna). The first OFDM component 56 includes aRF receiver 59, and an analog-to-digital converter 60, which convertsthe first received signal into digital signal samples. The signalsamples are passed to a frequency synchronizer 62 and a frequency offsetcorrector 64. The signal samples are also fed to a frame/timesynchronizer 66. Collectively, these three components producesynchronized signal samples.

The synchronized signal samples represent a time sequence of data. Thesynchronized signal samples are passed to a demultiplexer 68, thenpassed in parallel to a Fast Fourier Transform (FFT) processor 70. TheFFT processor 70 performs an FFT on the signal samples to generateestimated received symbols which are multiplexed in MUX 76 and sent asreceived symbols to decoder 78. Ideally, the received symbols would bethe same as the symbols fed into the IFFT processor 26 at the OFDMtransmitter 10. However, as the received signals will have likely beenaltered by the various propagation channels, the first OFDM component 56must correct the received symbols by taking into account the channels.The received symbols are passed to a channel estimator 72, whichanalyses received pilot symbols located at known times and frequencieswithin the OFDM frame. The channel estimator 72 compares the receivedpilot symbols with what the channel estimator 72 knows to be the valuesof the pilot symbols as transmitted by the OFDM transmitter 10, andgenerates an estimated channel response for each frequency and timewithin the OFDM symbol. The estimated channel responses are passed todecoder 78. The channel estimator 72 is described in detail below.

The second OFDM component 58 includes similar components as are includedin the first OFDM component 56, and processes the second received signalin the same manner as the first OFDM component 56 processes the firstreceived signal. Each OFDM component passes OFDM symbols to the decoder78.

The decoder 78 applies STBC decoding to the OFDM symbols, and passes thesymbols to a decoding/demodulating primitive 80 responsible fordecoding, de-interleaving, and demodulating the symbols to generateoutput binary data, as is well known to those skilled in the art. Thedecoding/demodulation primitive 80 which may include a number ofadditional processing blocks, not shown in FIG. 2. Each element in theOFDM components 56 and 58 is a processor, a component of a largerprocessor, or a collection of processors.

Referring now to FIG. 4, a method by which each of the pilot inserters24 and 40 of FIG. 2 inserts pilot symbols among the data symbols isshown. The method will be described with reference to the pilot inserter24 in the first OFDM component 20. At step 100, the pilot inserter 24receives data symbols from the demultiplexer 22. At step 102 the pilotSTBC function 23 generates (or receives) two pilot symbols. At step 104the pilot STBC function 23 applies STBC encoding to the pilot symbols,so as to generate an STBC block of encoded pilot symbols. The encodedpilot symbols generated for the first transmitting antenna 37 will beone row of the STBC block and will have a number equal to the number oftransmitting antennae in the OFDM transmitter. Thus, for a two antennasystem a 2×2 STBC block is generated.

At step 106 the pilot inserter 24 inserts the encoded pilot symbolswithin the OFDM symbol. Encoded pilot symbols are inserted in a diamondlattice pattern. The diamond lattice pattern uses the same frequenciesas the other diamond lattice patterns, but has a temporal offset fromthe other diamond lattice patterns. Preferably, the temporal offset foreach diamond lattice pattern is one symbol (in the time direction) fromanother diamond lattice pattern, so that the diamond lattice patternsuse consecutive symbols in the time direction of the OFDM frame.

The diamond lattice pattern in which each encoded pilot symbol isinserted within the OFDM frame is preferably a perfect diamond latticepattern. To achieve this, the encoded pilot symbol is inserted at eachof a first subset of frequencies. The frequencies within the firstsubset of frequencies are spaced equally apart by a pilot spacing. Theencoded pilot symbol is inserted at each of the first subset offrequencies for an STBC block (two OFDM symbols). At some later time,the encoded pilot symbols are inserted at each of a second subset offrequencies. The frequencies within the second subset of frequencies areshifted from the frequencies within the first subset of frequencies byhalf of the pilot spacing within the frequency direction. The pilotinserter 24 continues to insert encoded pilot symbols, alternatingbetween the first subset of frequencies and the second subset offrequencies.

Alternatively, a different pilot pattern can be used, as long as thesame pilot pattern is used for each of the at least one encoded pilotsymbols unique to the transmitting antenna 37, and as long as the pilotpatterns for the encoded pilot symbols are offset from each other in thetime direction of the OFDM frame. For example, a regular diagonallattice pattern may be used, the diamond shaped lattice being a specialcase of this.

The pilot inserter 40 inserts pilot symbols using the same method,although the pilot symbols will be the other half of the STBC block 42.The encoded pilot symbols unique to the second transmitting antenna 42are inserted in the OFDM frame at the same symbol locations at which theencoded pilot symbols corresponding to the first transmitting antenna 37are inserted.

Referring to FIG. 5, an example pilot pattern generated using the methodof FIG. 4 is shown. Pilot and data symbols are spread over the OFDMframe in a time direction 120 and a frequency direction 122. Mostsymbols within the OFDM frame are data symbols 124. A first set ofencoded pilot symbols 126 corresponding to the first transmittingantenna 37 is inserted in a diamond lattice pattern. A second set ofencoded pilot symbols 128 corresponding to the first transmittingantenna 37 is inserted in a diamond lattice structure at the samefrequencies as the first set of encoded pilot symbols, but offset by oneOFDM symbol location in the time direction 120. In the illustratedexample two of every four OFDM symbols carry encoded pilot symbols. Eachother transmitting antenna transmits using the same pattern. The pairsof consecutive pilot symbols on a sub-carrier consist of two raw pilotsymbols STBC encoded. The same pattern is transmitted by the secondantenna.

The power of the encoded pilot symbols 126, 128 may be increasedcompared to the traffic data symbol 124. The power increase of theencoded pilot can be dynamically adjusted with respect to thetransmitting data symbol power level or modulation type (QAM size), oras a function of channel quality. The location of diamond latticepattern may also be optimized to allow a fast extraction of scatteredpilot without using the computing. This may be achieved if the pilotsubcarriers are spaced in the frequency direction by 2^n. In themultiple base station transmission arrangement, the location of thediamond lattice pattern can be cyclic offset both in time direction andin frequency direction amongst adjacent base stations to form a diamondlattice re-use pattern.

Referring now to FIGS. 6 to 10, a channel estimation method is describedwhich is based on the pilot insertion method above. This inventionpresents a simple 2-dimensional channel interpolator for MIMO-OFDMsystem with low pilot density for fast fading channels both in time andin frequency. The goal of channel estimation is to estimate the channelcharacteristics for each sub-carrier and at each time for each possibletransmit antenna, receive antenna combination. Referring to FIG. 13, forthe two transmit antenna, two receive antenna example, shown are twotransmit antennas Tx1 140 and Tx2 142 and two receive antennas Rx1 144and Rx2 146. Channel estimation estimates a channel for each sub-carrierand at each time between Tx1 140 and Rx1 144 indicated as each H₁₁ 148,a channel between Tx1 140 and Rx2 146 indicated by transfer function H₁₂150, a channel estimate for transmitter Tx2 142 to Rx1 144 indicated astransfer function H₂₂ 152 and finally, a channel estimate fortransmitter Tx2 142 to receiver Rx2 146 indicated as transfer functionH₂₁ 154.

Some advantages for the proposed method compared to some existingmethods are: (1) robust to high mobility-speed (2) a reduction of thescattered pilot grid density and therefore a reduction of the pilotoverhead.

Let P₁ and P₂ be the two pilot symbols encoded in an STBC block andtransmitted by two antennas on one sub-carrier in consecutive OFDMsymbols. Then at the first receive antenna, the following relationshipexists for each sub-carrier on which pilot symbols are transmitted,where it is assumed the channel response H_(ij) is constant over twoOFDM frames:

$\begin{bmatrix}Y_{1,1} \\Y_{1,2}\end{bmatrix} = {\begin{bmatrix}P_{1} & P_{2} \\{- P_{2}^{*}} & P_{1}^{*}\end{bmatrix}\begin{bmatrix}H_{11} \\H_{21}\end{bmatrix}}$

Y_(1,1) is the received data on the first antenna on the sub-carrier inthe first of the two consecutive OFDM symbols, and Y_(1,2) is thereceived data on the first antenna on the sub-carrier in the second ofthe two consecutive symbols. This can be solved for H₁₁, H₂₁ to yield:

$\begin{bmatrix}H_{11} \\H_{21}\end{bmatrix} = {{\frac{1}{{P_{1}}^{2} + {P_{2}}^{2}}\begin{bmatrix}P_{1}^{*} & {- P_{2}} \\P_{2}^{*} & P_{1}\end{bmatrix}}\begin{bmatrix}Y_{1,1} \\Y_{1,2}\end{bmatrix}}$

A similar process for the second antenna yields

$\begin{bmatrix}H_{12} \\H_{22}\end{bmatrix} = {{\frac{1}{{P_{1}}^{2} + {P_{2}}^{2}}\begin{bmatrix}P_{1}^{*} & {- P_{2}} \\P_{2}^{*} & P_{1}\end{bmatrix}}\begin{bmatrix}Y_{2,1} \\Y_{2,2}\end{bmatrix}}$

where Y_(2,1) is the received data on the second antenna on thesub-carrier in the first of the two consecutive OFDM symbols, andY_(2,2) is the received data on the second antenna on the sub-carrier inthe second of the two consecutive OFDM symbols.

Using this techniques, a channel estimate is made for each pilotsub-carrier, and for each pair of OFDM symbols used to transmit STBCblocks.

For the example of FIG. 12, the result is a channel estimate, for eachof the possible channels (these are for channels in this example asshown in FIG. 13) for each pair of pilot symbols transmitted. This isillustrated in FIG. 14 where only sub-carriers used to transmit pilotsare shown. A channel estimate 150 is generated for each pair of(consecutive in time) OFDM frames for each pilot sub-carrier. Thisresults in channel estimates 150, 152, 154 for the first and secondframes, and channel estimates 156, 158, 160 for the fifth and sixthframes and so on.

The channel estimates are made on a STBC block by block basis so thatthe pattern of channel estimate shown in FIG. 7 develops over time. Thenext step in the process is to perform an interpolation based on thechannel estimate of FIG. 7 to obtain channel estimates for the places inFIG. 7 which do not represent pilot channel positions. The manner inwhich this is done will be described for a single example, namely theunknown channel estimate indicated at 163 of FIG. 7. Channel estimatesare buffered on an ongoing basis and when the four channel estimates152, 156, 158 and 164 forming a diamond 162 surrounding the unknownchannel estimate 163 have been computed, it is time to interpolate toobtain a channel estimate for the unknown point 163. The channeltransfer function at the sub-carrier located at the centre of thediamond can be obtained from a simple 4 points two-dimensionalinterpolator. Three points two-dimensional interpolators can be used toobtain the channel estimates corresponding to the first or last usefulsub-carrier:

$\begin{matrix}{{H_{new}( {{n + 1},k} )} = {\frac{1}{4}( {{H( {n,k} )} + {H( {{n + 2},k} )} +} }} \\ {{H( {{n + 1},{k - 1}} )} + {H( {{n + 1},{k + 1}} )}} ) \\{{where}\mspace{14mu}( {{k = 2},\ldots\mspace{14mu},{N_{pilot} - 1}} )} \\{{H_{new}( {{n + 1},1} )} = {\frac{1}{4}( {{H( {n,1} )} + {H( {{n + 2},1} )} +} }} \\ {2H( {{n + 1},2} )} ) \\{{H_{new}( {{n + 1},N_{pilot}} )} = {\frac{1}{4}( {{H( {n,N_{pilot}} )} + {H( {{n + 2},N_{pilot}} )} +} }} \\ {2H( {n,{N_{pilot} - 1}} )} )\end{matrix}$where k is the pilot sub-carrier index, n is the channel estimate index(or STBC block number—one channel estimate per sub-carrier for every twosymbols), and N_(pilot) is the number of pilot sub-carriers (6 in theexample of FIG. 7). H_(new) is the newly interpolated channel estimatefor the i^(th) channel estimation period, and the j^(th) pilotsub-carrier. E(i, j) is the channel estimate determined as describedpreviously from the pilot symbols. A three points interpolator wouldalso be performed for the last STBC blocks in the OFDM frame (i.e. thelast two OFDM symbols).

These calculations are done for each transmit antenna, receiver antennacombination. It is noted that this is just one example of how thechannel estimates can be interpolated.

If the original distance between pilot sub-carriers in the frequencydirection is D_(f), after first step of interpolation described above,the pilot sub-carriers' separation becomes

$\frac{D_{f}}{2}.$

In some embodiments, to remove noise, the channel estimates thuscomputed are filtered at each channel estimation period. This is shownin FIG. 6 where the channel estimates 170 for one channel estimationperiod are shown entering filter 172 to produce filtered channelestimates. For example, a simple 3 point moving iterative smoothingalgorithm may be applied to H′:H′ _(sm)(n,k)=H′ _(sm)(n,k−1)+⅓(H′(n,k+1)+H′ _(sm)(n,k−2))

where k=3, . . . , 2 N_(pilot)−2. It is to be understood that otherfiltering algorithms may be employed.

After the interpolation of the pilot channel estimate as summarized inFIG. 7, there will be a channel estimate for each sub-carrier on whichpilot channel information was transmitted and for each two OFDM symbolperiod over which pilot channeling information was transmitted.Referring to FIG. 5, this means that there will be a channel estimatefor each antenna for time frequency points which are shaded to indicatethat pilot channel information was transmitted. There will also bechannel estimates for the time frequency point in the centre of thediamond shaped lattice structure of FIG. 7. However, for points whichare not pilot symbol transmission time-frequency points nor points whichare at the centre of a diamond shaped lattice of such points, there willbe no channel estimate yet computed. The next step is to perform afurther interpolation step to develop channel estimates for these otherpoints.

In some embodiments, Cubic Lagrange interpolation and linearinterpolation (for the sub-carriers near the first and the last usefulsub-carrier) in the frequency direction are used to obtain the channeltransfer function at all sub-carriers for each STBC block (for each pairof OFDM symbols).

The coefficients of the Cubic Lagrange interpolator can be calculated as

$\begin{matrix}{{\mu(i)} = \frac{i}{D_{f}/2}} \\{{i = 1},2,\ldots\mspace{14mu},\frac{D_{f}}{2}} \\{{q_{- 1}(\mu)} = {{{- \frac{1}{6}}\mu^{3}} + {\frac{1}{2}\mu^{2}} - {\frac{1}{3}\mu}}} \\{{q_{0}(\mu)} = {{\frac{1}{2}\mu^{3}} - \mu^{2} - {\frac{1}{2}\mu} + 1}} \\{{q_{1}(\mu)} = {{{- \frac{1}{2}}\mu^{3}} + {\frac{1}{2}\mu^{2}} + \mu}} \\{{q_{2}(\mu)} = {{{- \frac{1}{6}}\mu^{3}} - {\frac{1}{6}\mu}}}\end{matrix}$

The channel transfer functions at data sub-carriers are given by

${H_{interp}( {{( {j - 1} ) \cdot \frac{D_{f}}{2}} + i} )} = {\sum\limits_{n = {- 1}}^{2}\;{{{q_{n\;}( {\mu(i)} )} \cdot ( H^{\prime} )_{sm}}( {j + n} )}}$where  j = 2, …  , N_(pilot) − 2.

This is illustrated in FIG. 9 where the estimated channel responses arefed to the Legrange cubic interpolator function 175 which outputs valuesfor all intermediate sub-carriers. Other interpolations mayalternatively be employed.

In some embodiments, every OFDM symbol contains some pilot insertionpoints and as such this completes the interpolation process. In otherembodiments, there are some OFDM symbols which do not have any pilotinsertion points. To get channel estimates for these OFDM symbols, aninterpolation in time of the previously computed channel estimates isperformed. In high mobility applications, pilots should be included inevery OFDM symbol avoiding the need for this last interpolation in timestep.

FIG. 10 presents an overall block diagram of the interpolation methodproposed for two transmit antennas. An example set of performanceresults for the proposed MIMO-OFDM channel estimation algorithm is shownin FIG. 10. The performance of the 2-D channel estimation algorithm isclose to the performance of ideal channel (only 0.5 dB loss) at veryhigh Doppler spread.

Referring now to FIGS. 10 and 3, the channel estimation method iscarried out by the channel estimator 72 in order to estimate a channelresponse for each sub-carrier and each OFDM symbol within an OFDM frame.The channel estimation method starts at step 500 by extracting the pilotsymbols in the frequency domain for each receive antenna. This followedby a channel response matrix computing step 502; whereby the receivedsignal received by the receiving antenna is decoded, which in effectperforms a time-average of the encoded pilot symbols at each point inthe pilot pattern. For example, suppose the receiving antenna receivesan OFDM frame having a pilot pattern as shown in FIG. 5 (although thesymbol 126 will now be a linear combination of the encoded pilot symboltransmitted at this location by each of the transmitting antenna, andthe symbol 128 will be a linear combination of the encoded pilot symboltransmitted at this location by each of the transmitting antenna).Following decoding, the pilot symbol at symbol location 126 will be anaverage of the pilot symbol received at symbol location 126 and thepilot symbol received at symbol location 128. The time averaging effectproduced by the STBC decoding, during step 503, can be viewed as apre-processing step, as can steps 500 and 502. The actual channelestimation method can be described broadly in four steps. Following step503, during step 504 the channel estimator 72 estimates the channelresponse for each of a plurality of pilot symbols. For a diamond latticepattern, the plurality of pilot symbols will be four pilot symbolsforming a single diamond pattern. The channel estimator 72 estimates thechannel response of a central symbol, the central symbol having a timedirection value and a frequency direction value bounded by the timedirection values and the frequency direction values of the plurality ofpilot symbols. The central symbol preferably has a frequency directionvalue equal to the frequency direction values of two of the plurality ofpilot symbols, and has a time direction value midway between the timedirection values of the two pilot symbols having the same frequencydirection value as the central symbol. This can generally be describedas a four-point 2-D interpolation of the channel response between pilotsymbols. Third, the channel estimator 72 smoothes the channel responses(corresponding to both encoded pilot symbols and to the central symbol)in the frequency direction, preferably by performing a three-pointsmoothing, as per step 505. Fourth, the channel estimator 72 performs aninterpolation in the frequency direction to estimate the channelresponse for remaining symbols, as per step 506. The interpolation maybe a linear interpolation for symbols having a frequency direction valueequal to a first or a last useful sub-carrier within the OFDM symbol,and a cubic Lagrange interpolation otherwise.

The method of inserting pilot symbols (described above with reference toFIG. 4) and the channel estimation method (described above withreference to FIG. 10) need not be used together. Any channel estimationmethod may be used by the OFDM receiver to estimate the channelresponses for an OFDM frame containing encoded pilot symbols insertedusing the method described above. However, due to the sparsedistribution of the pilot symbols in the pilot pattern described abovewith reference to FIG. 4 and FIG. 5, a two-dimensional interpolationmethod is preferable over a one-dimensional interpolation method.Similarly, the channel estimation method may be applied to an OFDM framecontaining any pattern of pilot symbols.

The invention has been described with respect to an MIMO-OFDMcommunication system. The invention may also be used with advantage in asingle input-multiple output OFDM communication system, as the method ofinserting pilot symbols (described with reference to FIG. 4) and thechannel estimation method (described with reference to FIG. 10) do notdepend on the number of receiving antenna. Each receiving antenna withinthe OFDM receiver 50 performs channel estimation independently,regardless of the number of receiving antennae present.

The channel estimation method described with reference to FIG. 10 willalso be advantageous in an OFDM communication system having only onetransmitting antenna, as the method provides an improved interpolationof the channel response regardless of the number of transmittingantenna. The method of inserting pilot symbols described with referenceto FIG. 11 may be used in an OFDM communication system having only onetransmitting antenna, but will not be as advantageous as in an OFDMcommunication system having more than one transmitting antenna as therewill be no reduction in overhead.

The method of inserting pilot symbols and the channel estimation methodare preferably implemented on the OFDM transmitter and on the OFDMreceiver respectively in the form of software instructions readable by adigital signal processor. Alternatively, the methods may be implementedas logic circuitry within an integrated circuit. More generally, anycomputing apparatus containing logic for executing the describedfunctionality may implement the methods. The computing apparatus whichimplements the methods (in particular the pilot inserter or the channelestimator) may be a single processor, more than one processor, or acomponent of a larger processor. The logic may comprise externalinstructions stored on a computer-readable medium, or may compriseinternal circuitry.

What has been described is merely illustrative of the application of theprinciples of the invention. Other arrangements and methods can beimplemented by those skilled in the art without departing from thespirit and scope of the present invention.

1. A method of inserting pilot symbols into Orthogonal FrequencyDivision Multiplexing (OFDM) frames transmitted on a plurality N oftransmitting antennas, N>=2, the OFDM frames having a time domain and afrequency domain, each OFDM frame comprising a plurality of OFDMsymbols, the method comprising the steps of: for the N transmitantennas, transmitting sets of N pilot symbols, each set being in alocation within a scattered pattern in time-frequency, each set of Npilot symbols comprising a pilot symbol for each antenna, wherein forthe N transmit antennas, transmitting sets of N pilot symbols, each setbeing in a location within a scattered pattern in time-frequencycomprises: inserting sets of N pilot symbols at locations that form atleast one diagonal arrangement in time-frequency, wherein inserting setsof N pilot symbols comprises: when N is equal to two, for each antenna,alternating insertion of null symbol locations and pilot symbols in theat least one diagonal arrangement for a first antenna of the pair ofantennas and alternating insertion of pilot symbols and null symbollocations in the at least one diagonal arrangement for a second antennaof the pair of antennas, wherein the null symbol locations of the firstantenna correspond to a same location in time-frequency as the pilotsymbols of the second antenna, and vice versa.
 2. The method of claim 1wherein transmitting sets of N pilot symbols, each set being in alocation within a scattered pattern in time-frequency comprises:transmitting a set of N pilot symbols in a respective location withinthe scattered pattern on a same sub-carrier.
 3. The method of claim 1further comprising transmitting the sets of N pilot symbols with a powerlevel greater than a power level of data symbols, depending upon a valuereflective of channel conditions.
 4. The method of claim 1 furthercomprising transmitting the sets of N pilot symbols with a power levelwhich is dynamically adjusted to ensure sufficiently accurate receptionas a function of a modulation type applied to sub-carriers carryingdata.
 5. The method of claim 1 further comprising inserting sets of Npilot symbols in an OFDM resource for an additional group of Ntransmitting antennas wherein transmitting sets of N pilot symbols in arespective pattern in time-frequency for the additional group of Ntransmitting antennas comprises: employing the same respective patternof pilot symbols as the N transmitting antennas where N≧2, but offset inat least one of time and frequency.
 6. A method comprising: a firsttransmitter implementing the method of claim 1; at least one othertransmitter implementing the method of claim 4 using scattered patternsoffset from those used by the first transmitter.
 7. The transmitter ofclaim 1 wherein a set of N pilot symbols in a respective location withinthe scattered pattern is transmitted on a same sub-carrier.
 8. A methodof inserting pilot symbols into Orthogonal Frequency DivisionMultiplexing (OFDM) frames transmitted on a plurality N of transmittingantennas, N>=2, the OFDM frames having a time domain and a frequencydomain, each OFDM frame comprising a plurality of OFDM symbols, themethod comprising the steps of: for the N transmit antennas,transmitting sets of N pilot symbols, each set being in a locationwithin a scattered pattern in time-frequency, each set of N pilotsymbols comprising a pilot symbol for each antenna, for each locationwithin a scattered pattern in time-frequency: generating a group of Luncoded pilot symbols; performing space time block coding (STBC) on thegroup of L uncoded pilot symbols to produce an N×N STBC block, L and Ndetermining an STBC code rate; transmitting one row or column of theSTBC block on each antenna on a specific sub-carrier.
 9. A method ofinserting pilot symbols into Orthogonal Frequency Division Multiplexing(OFDM) frames transmitted on a plurality N of transmitting antennas,N>=2, the OFDM frames having a time domain and a frequency domain, eachOFDM frame comprising a plurality of OFDM symbols, the method comprisingthe steps of: for the N transmit antennas, transmitting sets of N pilotsymbols, each set being in a location within a scattered pattern intime-frequency, each set of N pilot symbols comprising a pilot symbolfor each antenna, wherein transmitting sets of N pilot symbols, each setbeing in a location within a scattered pattern in time-frequencycomprises: providing a first plurality of equally spaced sub-carrierpositions; providing a second plurality of equally spaced sub-carrierpositions offset from said first plurality; inserting the sets of Npilot symbols alternately in time using the first plurality of equallyspaced sub-carrier positions and the second plurality of equally spacedsub-carrier positions.
 10. The method of claim 9 wherein the secondplurality of equally spaced sub-carrier positions is offset from thefirst plurality of equally spaced-subcarrier positions by half thespacing between adjacent sub-carriers of the first plurality ofsub-carrier positions thereby forming a diamond shaped arrangement. 11.A transmitter comprising: a plurality N of transmit antennas, N>=2; anOFDM frame generator that inserts pilot symbols into OrthogonalFrequency Division Multiplexing (OFDM) frames transmitted on theplurality N of transmit antennas, the OFDM frames having a time domainand a frequency domain, each OFDM frame comprising a plurality of OFDMsymbols, such that for the N transmit antennas, sets of N pilot symbolsare transmitted, each set being in a location within a scattered patternin time-frequency, each set of N pilot symbols comprising a pilot symbolfor each antenna; the transmitter further operable to, for each locationin the scattered pattern: generate a group of L uncoded pilot symbols;perform space time block coding (STBC) on the group of L pilot symbolsto produce an N×N STBC block; transmit one row or column of the STBCblock on each antenna.
 12. The transmitter of claim 11 further operableto transmit the sets of N pilot symbols with a power level greater thana power level of data symbols depending on a value reflective of channelconditions.
 13. The transmitter of claim 11 further operable to transmitthe sets of N pilot symbols with a power level which is dynamicallyadjusted to ensure sufficiently accurate reception.
 14. A transmittercomprising: a plurality N of transmit antennas. N>=2; an OFDM framegenerator that inserts pilot symbols into Orthogonal Frequency DivisionMultiplexing (OFDM) frames transmitted on the plurality N of transmitantennas, the OFDM frames having a time domain and a frequency domain,each OFDM frame comprising a plurality of OFDM symbols, such that forthe N transmit antennas, sets of N pilot symbols are transmitted, eachset being in a location within a scattered pattern in time-frequency,each set of N pilot symbols comprising a pilot symbol for each antenna,wherein the OFDM frame generator is operable to: define a firstplurality of equally spaced sub-carrier locations; define a secondplurality of equally spaced sub-carrier locations offset from said firstplurality; wherein the sets of N pilot symbols are inserted alternatelyin time using the first plurality of equally spaced sub-carrierlocations and the second plurality of equally spaced sub-carrierlocations.
 15. The transmitter of claim 14 wherein spacing betweenlocations of the scattered pattern in time-frequency is optimized toallow a fast extraction of scattered pilot symbols without requiring thecomputation of a complete FFT.